Alterations in mitochondrial activity resulting from defects in mitochond-rial DNA (mtDNA) can modulate the biogenesis of mitochondria by mech-anisms that are still poorly understood. In order to study mitochondrial biogenesis in cells with impaired mitochondrial activity, we used rho-L929 and rho 0 143 B cells (partially and totally depleted of mtDNA, respectively),